In nuclear reactors, there is generally provided a so-called primary shutoff system which can include control rods or shutoff rods which can be inserted to various degrees into the reactor core and consisting of a neutron-absorptive solid material.
The shutdown of a nuclear reactor is generally a result in the interruption of the fission reaction by absorbing a substantial part of the neutron flux by such control or shutdown rods which prevent the absorbed neutrons from participating in further chain reactions.
To increase the safety of the reactor, at least one further shutdown system is generally provided, the further or second shutdown system being operated independently from the first and being designed to reduce the neutron flux and thereby limit the reactivity of the nuclear reactor.
In most existing systems, the secondary shutdown system, like the primary shutdown system, is characterized by the possibility that, after elimination of the emergency condition which triggered its operation, the reactor activity can build up again.
Naturally, this should not be the case for an emergency shutdown system intended to prevent reactivation of the nuclear reactor, for example, when the power-generating unit of a power-generating nuclear reactor fails. Hence it is desirable to provide a secondary shutdown system which acts practically irreversibly in certain situations but reversibly in others.
There are a number of processes which have been used heretofore for the shutdown of nuclear reactor installations. For example, it has been proposed in German Patent Publication (Auslegeschrift) DT-AS No. 14 89 832 with means for introducing into the liquid-metal coolant, a metal soluble therein and having a large neutron-absorption cross section. For sodium-cooled nuclear reactors, the additive is lithium (Li), rubidium (Rb) and cesium (Cs). The neutron-absorbing agent can be introduced into the liquid-metal coolant to shut down the nuclear reactor ahead of the inlet for the coolant into the reactor core via venturi nozzlers (see German Patent Publication-Offenlegungsschrift-No. 21 14 008).
These additives can be removed from the sodium melt by skimming from the surface.
For a fresh startup, the sodium melt is treated with nitrogen. Experience has shown that with the use of lithium as an additive, an insoluble lithium nitrogen compound forms in the sodium melt and makes the startup of the nuclear reactor difficult.
For gas-cooled nuclear reactors, it is proposed in German Patent Publication (Offenlegungsschrift) No. 18 04 336 to flood the nuclear reactor core with neutron-absorbing liquids through a pipe system. Such a pipe system is, however, difficult to provide in most gas-cooled nuclear reactors. Mention should also be made of the fact that it is known to flood control rods which are insertable into the reactor core (see German Patent Publication-Offenlegungsschrift-DT-OS No. 18 01 395). The difficulty with the latter system is that the secondary shutdown arrangement is not independent from the functioning of the control rods and hence cannot be properly used as a secondary safety or shutdown arrangement in the manner described previously.
From German Patent Publication (Offenlegungsschrift) DT-OS No. 18 09 399, there is known a system for shutting down a gas-cooled nuclear reactor by the introduction of neutron-absorbing material in solid form over separate pipes provided in the interior of the reactor core. The installation of special pipes for this purpose, however, poses a problem in most gas-cooled nuclear reactors because of the structural or technological disadvantages. These disadvantages can be avoided with a shutdown process of the type described in German Patent Publication (Offenlegungsschrift) No. 25 16 123, in which, in an emergency, the reactor is flooded with de-ionized water to which the absorbing agent has been added. A process of the latter type can, however, only be used at low temperatures since at higher temperatures substantial steam generation must be reckoned with and damage to other units of the nuclear reactor must be considered, especially when the primary cooling cycle includes turbines. The flooding with water is therefore used only in the most extreme emergencies.
German Patent Publication (Auslegeschrift) No. 10 69 303 proposes the addition to the cooling medium of neutron-absorbing metal particles, for example cadmium particles. German Patent Publication (Offenlegungsschrift) DT-OS No. 23 25 828 describes a system in which a circulating-ball reactor is shut down by the addition of balls of absorbing material to the reactor core, the absorber balls being introduced into the interstices between the nuclear-fuel balls of the reactor core. The introduction into the coolant of metal particles or the introduction into the reactor core of absorber balls permit removal only upon withdrawal of at least a portion of the fuel elements from the reactor core. To facilitate removal of the absorber balls, for example, special reactor core bottoms must be provided.
Mention should also be made of the system described in German Patent Publication (Auslegeschrift) DT-AS No. 11 23 415 which describes the admixture to the coolant of a neutron-absorbing gas, for example, boron trifluoride (BF.sub.3). Such gases are dispersed, after introduction into the coolant, over the entire cooling cycle so that large gas quantities must be used to shut down the nuclear reactor and considerable difficulty is created in removing the absorptive gas from the cooling-gas stream subsequently. The use of BF.sub.3, moreover, results in deterioration of the reactor system because of the high corrosivity of this agent. Here again, the use of this method is practical only in the most dangerous or emergency conditions.